(1) Field of the Invention
This invention relates to a process for preparing fine particles of ferromagnetic iron compounds, particularly ferromagnetic iron oxides for use in magnetic recording such as Fe.sub.3 O.sub.4, .gamma.-Fe.sub.2 O.sub.3, or .alpha.-Fe metal.
(2) Description of the Prior Art
Magnetic characteristics required for magnetic materials used in magnetic recording are that they have a high magnetic coercive force, that both saturated susceptibility (.sup..sigma. s) and residual susceptibility (.sup..sigma. r) are high, and that the squareness ratio (R=r/.sup..sigma. s) is large. It is advantageous for iron series ferromagnetic compounds to have a fine needle-like particle form in order to satisfy such magnetic characteristics. Various processes for the preparation of needle-like iron oxyhydroxide particles or needle-like iron oxide particles have been disclosed. For example, Japaenese Pat. No. 166146 discloses a process for preparing needle-like iron oxyhydroxide particles in which FeSO.sub.4.7H.sub.2 O as a starting material is neutralized with NaOH, and then subjected to air oxidation and a seeding crystallization process.
According to the conventional processes for the preparation of needle-like iron compounds, however, in the case where (a) fine needle-like iron oxyhydroxide particles are subjected to catalytic reaction with a reducing gas to form needle-like Fe.sub.3 O.sub.4 particles, (b) the needle-like Fe.sub.3 O.sub.4 particles are treated with an oxidizing gas to form needle-like .gamma.-Fe.sub.2 O.sub.3, and (c) fine needle-like iron oxyhydroxide particles and/or iron oxide particles are subjected to catalytic reaction with a reducing gas to form .alpha.-Fe particles, the catalytic reaction of these particles as starting materials with an oxidizing gas and/or reducing gas inevitably causes the breakage and sintering of the particles even if the majority of these particles has a fine needle-like form. Consequently, marked deterioration in magnetic characteristics of the particles results. There is a lowering in magnetic coercive force, saturated susceptibility, residual susceptibility, and squareness ratio such that the properties required for the ferromagnetic iron compound for use in magnetic recording are greatly impaired.
For the purpose of overcoming the problems in the conventional processes as above, for example, Japanese Patent Laid-Open Publication No. 122663/'79 discloses a process for preparing iron compound particles for use in magnetic recording which comprises cladding iron oxyhydroxide, iron oxide, or a composition prepared by doping thereto a metal such as cobalt, manganese, or nickel with a compound of one or more than one of zinc, chromium, and copper, drying, and reducing the resulting product at a temperature of from 200.degree. to 600.degree. C. under a reducing gas stream. The problems in the conventional processes described above, however, can not be solved satisfactorily even if the above process is effected.